The 21-year-old was introduced in the 38th minute after the Italian suffered a shoulder injury, as noted by the club's official Twitter feed:
Manchester United @ManUtd
38' - Substitution for #mufc: Donald Love makes his United debut, replacing the injured Matteo Darmian.2/13/2016, 1:22:43 PM
Love had been on loan at Wigan Athletic earlier in the season and has represented Scotland under-21s on one occasion. He was named on a young substitutes' bench by manager Louis van Gaal for the game against the Black Cats.
As noted by Squawka Football's Greg Johnson, it almost seemed inevitable that Love would get an outing on the eve of Valentine's Day:
Greg Johnson @gregianjohnson
Of course Manchester United weren't going to start their comeback until Donald Love was on the field on the weekend of Valentines Day.2/13/2016, 1:25:27 PM
According to Liam Prenderville of the Daily Mirror, although Love is most comfortable as a right-back, he is a versatile player.
"The 21-year-old has played in the centre of defence and left-back for United's youth sides, as well as filling in in midfield," he wrote. "One of Love's key attributes is his athleticism and ability to cope with attackers in one-vs-one situations."
It's also noted that the youngster has been at United since he was seven. He signed his first professional contract with the club in 2013, but he has had his development halted by a succession of injury issues.
Needless to say, after spending so long at the club, getting on to the field with the senior side will be a very proud moment for the young Scotsman.